The present invention relates to an injection device for a liquid fuel.
Such an injection device is generally known in the prior art. injection of a liquid fuel has the advantage that a relatively small quantity of fuel has to be introduced per injection, so that a relatively compact injector will suffice. Moreover, when an alternative fuel such as LPG is being injected, in which case the installation concerned is fitted later, as is often the case with motor vehicles, liquid introduction is advantageous because use can be made of the control system for the original fuel injection present in such a vehicle, generally petrol injection.
However, a problem is that during the injection of such a liquid fuel into the inlet duct of the internal combustion engine a large quantity of heat is extracted during the evaporation.
In the first instance, this led to blockages of the outlet pipe of the injector because of freezing of moisture present in the air. After this problem had been overcome, inter alia through the use of a curved outlet pipe extending over a considerable distance in the outlet manifold, although no blockage of the outlet opening occurred, the problem was that moisture still froze on said outlet pipe. If a particular vehicle is under less load, the frozen ice deposit will come away from the pipe and be entrained in the stream of inlet air. This can cause considerable damage in particular to the inlet valve or valve seat, while in any case a disturbance in the operation of the engine concerned will be experienced in the form of a shock while the ice is coming away.
The object of the present invention is to avoid this disadvantage and to provide an injector with an outlet in which, on the one hand, the opening does not freeze up during the outflow of the liquid, rapidly evaporating fuel and, on the other hand, no ice deposits occur on the outlet.
The invention is based on the idea of, on the one hand, making the external part of the outlet pipe of a heat-conducting material, so that the temperature thereof during injection of the liquid fuel into the outlet pipe does not fall to such an extent that moisture can form an ice deposit. On the other hand, the interior of the pipe is made of an insulating material, so that the risk of premature evaporation of the liquid fuel does not occur and possibly lead to blockage of the outlet opening.
It is pointed out that the PCT application WO 89/04920 discloses an injection valve wherein a sliding pin made of metallic material is received in a ceramic tube. At the bottom end the pin seals off at the end of the ceramic tube. The pin is extended beyond the seal, as is the ceramic material part. This means that it will be necessary during operation to add heat to the pin, otherwise the freezing phenomenon described above will inevitably occur.
The construction according to the invention is perfected if the outlet opening of the heat-insulating pipe is fitted substantially flush with the heat-conducting material. This limits to a minimum the surface where problems might occur.
Optimum insulation between the insulating pipe and the heat-conducting material can be obtained if an empty space is provided between them. For this purpose, the heat-conducting material preferably contains a sleeve.
The outlet pipe is preferably provided with a curvature, in order to ensure that the injection of the fuel occurs extending substantially parallel to the longitudinal axis of the inlet manifold.
Plastic material is found to be extremely satisfactory as the insulating material, while a metallic material, more particularly brass, is preferably used as the heat-conducting material.